Uh, the weather sort of sucks, but Millenium park is a decent way to kill an hour or two - The Bean is one of the cooler works of public art around. If you're into museums, Chicago's Institute of Art is really good, and has several world-famous paintings. It's also right next to Millenium Park, so if you do one, you can easily do the other. You're in the loop - and I know that there is a land-bound architecture tour (the one on the boat is more popular) of the older "skyscrapers" that abound in that area of town.
If you're around tomorrow, you can always go watch the Chicago Polar Bear club go swimming at North Ave. Beach.